Field of the Invention
The present invention relates to a core of a magnetic head for writing, reading and erasing information in video tape recorders, magnetic disk drives and other information recording and reproduction apparatus, and a method for producing such a magnetic head core. More particularly, the invention is concerned with an improved magnetic head core made of a ferrite material, which provides increased bonding strength at the bonded interface of two ferrite core elements and which has a magnetic head gap whose width is accurately controlled during manufacture.
Discussion of the Prior Art
In the art of magnetic heads used with various types of data storage media, there has been known a core made of ferrite, which generally has a pair of ferrite core elements butted and joined together so as to form a toroidal or doughnut-shaped core having a centrally located aperture or space, which facilitates or permits winding of coils around the core elements. More specifically described referring to FIG. 1 showing a typical known type of magnetic head core, a pair of generally C-shaped ferrite core halves or elements 2, 4 are butted together to form a core with a central aperture 6 which serves as a space for winding coils 8 around the core elements 2, 4. Thus, the two ferrite core elements 2, 4 cooperate with each other to constitute a generally annular or toroidal magnetic path or circuit. The toroidal magnetic head core structure has a magnetic gap 10 formed so as to extend across the annulus or torus of the core or magnetic path. The magnetic gap 10 has a predetermined suitable width .alpha. which is a distance between the opposed end faces of the joined core halves 2, 4 at their corresponding ends. As is well known in the art, a magnetic tape 12, magnetic disk or other magnetic recording medium is slidably moved on the outer contact surface formed on a portion of the magnetic head core 2, 4 in which the magnetic gap 10 is formed. Thus, information is written or recorded on and read or retrieved from the recording medium, through the magnetic head core.
As indicated above, one of the opposite ends of the ferrite core element 2 is spaced apart from the corresponding end of the other ferrite core element 4, by a distance equal to the width .alpha. of the magnetic gap 10, while the other corresponding ends of the core elements 2, 4 are bonded together with a suitable bonding glass, as indicated at 14 in FIG. 1, such that the two core elements 2, 4 maintain the generally annular or doughnut-like cross sectional configuration to provide a generally toroidal magnetic circuit.
In the known ferrite magnetic head core wherein the core elements 2, 4 are bonded together with a glass, the bonding strength at the bonded portion 14 is not sufficiently high, and the head core 2, 4 may be broken or otherwise damaged at the bonded portion 14 in the subsequent steps of manufacture, for example, in the process of winding the coils 8 around the core elements 2, 4.
The magnetic head core is further subjected to heat treatments for various purposes, including a heat treatment for filling the magnetic gap 10 with a glass filler. During the heat treatments, the glass material at the bonded portion 14 may be softened. This means a potential problem that the width .alpha. of the magnetic gap 10 may deviate from the predetermined nominal value To prevent this problem, the bonding glass for the bonded portion 14 and the glass filler should have different softening points.
Usually, the bonding of the two ferrite core halves or elements 2, 4 as used in the head core of FIG. 1 is conducted by using a fixture as illustrated in FIG. 2. Namely, the butted two core elements 2, 4 are held in position on the fixture, and suitable bonding glass and glass filler 16 are applied to the bonding surfaces 14 and a magnetic gap portion 18 of the butted core elements 2, 4. If the pressure is not evenly exerted to the bonding surfaces 14 of the butted core elements 2, 4, the width of the magentic gap 10 obtained may significantly fluctuate with respect to the predetermined optimum value.
In an attempt to solve the above-indicated potential problem experienced in the prior art, the assignee of the present application proposed a magnetic head core and a method for producing the same, as disclosed in laid-open Publication No. 60-138710 (published in 1985) of unexamined Japanese patent application. According to the proposed head core and method, a generally annular ferrite core structure is formed by a plurality of ferrite core elements which are bonded together by direct solid-solid reaction bonding at a portion of the structure remote from the magnetic gap portion.
The method proposed in the above-identified Publication is schematically illustrated in FIG. 3, wherein a first ferrite core element 22 as one of two elements of the head core to be produced is subjected to a process of forming a groove 26 and a recess 28 such that the groove 26 and the recess 28 define therebetween a gap-defining surface 30. This gap-defining surface 30 is then chemically etched to a suitable depth 8 corresponding to the desired width .alpha. of the magnetic gap to be obtained. The thus prepared first ferrite core element 22 and a second ferrite core element 24 are butted together such that the etched gap-defining surface 30 of the first core element 22 is spaced apart from the corresponding surface of the second core element 24, by a distance equal to the magnetic gap width .alpha. (etching depth .beta.). The two core elements 22, 24 are bonded together at the corresponding end portions remote from the gap-defining surface 30, by means of direct solid-solid reaction bonding Thus, an integral ferrite structure 32 consisting of the bonded ferrite core elements 22, 24 is obtained. The ferrite structure 32 has a magnetic gap 36 filled with a glass filler 38, and a coil-winding aperture 34 defined by the recess 28 of the first core element 22 and the corresponding surface of the second core element 24. The ferrite structure 32 is then subjected to an operation to remove its end portion remote from the bonded ends of the core elements 22, 24, so that the magnetic gap 36 filled with the glass filler 38 is exposed to a suitably formed outer contact surface 42. In this manner, a ferrite head core 40 is prepared.
For effecting the solid-solid reaction bonding, the two ferrite core elements 22, 24 are heated to a relatively high temperature of about 1100.degree.-1250.degree. C., according to the above-identified Publication. During this heating process for the solid-solid reaction bonding, not only the outer surfaces of the ferrite core elements 22, 24 but also the inner surfaces defining the magnetic gap width .alpha. are subjected to evaporation of zinc of the ferrite and oxidation-reduction reaction, due to an oxygen partial pressure of an atmosphere surrounding the ferrite core elements. As a result, the width .alpha. of the magnetic gap 36 may have an error of .+-.0.1 micron or more with respect to the nominal value.
Recently, there is a tendency toward increased data storage density per unit area of a recording medium. This tendency requires the magnetic head core to have an accordingly reduced magnetic gap width and accordingly improved accuracy of the gap width. For example, the currently available VTR magnetic head core requires the magnetic gap width of 0.4 micron with a permissible error of .+-.0.03 micron, and the RDD (rigid magnetic disk drive) magnetic head core requires the magnetic gap width of 0.8 micron with a permissible error of .+-.0.08 micron.